Swimmer

A swimmer is any monster that has the M1_SWIM flag set. Only swimmers will be generated on the Plane of Water.[1]
Swimming is largely similar to amphibiousness, preventing monster from drowning. One difference is that swimming monsters can pick up items underwater[2]. Attacking a swimmer with a trident while it is in water gives a +4 to-hit bonus.[3]

Entire classes

Unique monsters

You

Swimming property can be obtained by polymorphing into one of the above monsters or riding a swimming steed. It is very similar to magical breathing, allowing you to stay in water but not preventing water damage. Interestingly, while messages imply that you stay on the surface of water, you still cannot see your surroundings while swimming[4].